Make tag suggestion whenever possible (ala taglocity)
Files have characteristics (eg: extension) and many files have ascii content that can be parsed somehow.
Having tag suggestion based on file characteristics/content would be a real plus for people having to massively tag files.
Taglocity did it really well in its Firefox extension.
They used machine learning to improve their suggestion engine.
Even if you restrict this to pdf/txt/doc/docx, it would be fantastic.